1. Field of the Invention
The present invention relates to a liquid crystal projector for projecting images displayed on a liquid crystal display onto a translucent screen.
2. Description of the Related Art
Recently, as a display for a liquid-crystal television receiver using a dot-matrix liquid crystal display having a small screen, liquid crystal projectors have been developed which project displayed images of the liquid crystal display on a large screen so as to enlarge the small images displayed on the liquid crystal display.
In the liquid crystal projectors, light is directed from an illumination lamp to a transmissive dot-matrix liquid crystal display, and light transmitted through the liquid crystal display, which corresponds to a displayed image of the liquid crystal display, is projected on a screen through a projection lens. In general, the liquid crystal display utilizes twisted nematic (TN) liquid crystals.
As is well known, the liquid crystal display utilizing twisted nematic (TN) liquid crystals is formed as follows: the nematic liquid crystals are sealed in, in twisted arrangements of substantially 90 degrees, between a pair of transparent substrates, on the opposed surfaces of which electrodes are formed, to form a TN liquid crystal panel; and a polarizing plate is provided on each of light-falling and light-outgoing planes of the TN liquid crystal panel. The polarizing plate on the light-falling plane of the liquid crystal panel is adapted to allow only light oscillating in a predetermined direction, emitted by the illumination lamp, to enter the liquid crystal panel. The liquid crystals sealed in the liquid panel are twisted with reference to the direction of the axis of passage of light of the light-falling polarizing plate. On the other hand, the polarizing plate on the light-outgoing plane of the liquid crystal panel is adapted to allow the passage of light transmitted through the liquid crystal panel and oscillating in a predetermined direction, thereby forming an optical image. Where the liquid crystal display is of a normally black type in which portions to which an electric field is applied can transmit light, while portions to which no electric field is applied interrupt light so as to display images, the axis of passage of light of the image-forming polarizing plate on the light-outgoing plane is made parallel to that of the polarizing plate on the light-falling plane. On the other hand, in the case of a normally white type in which portions to which an electric field is applied interrupt light, while portions to which no electric field is applied can transmit light, the image-forming polarizing plate has its axis of passage of light made substantially normal to that of the light-falling polarizing plate.
The conventional liquid crystal display uses, as the TN type liquid crystal display, one used in a direct-vision type display in which the light-outgoing plane is observed directly. In the TN type liquid crystal display used in the direct-vision type display, the light-falling polarizing plate and the image-forming polarizing plate on the light-outgoing plane have their axes of passage of light arranged to make an angle of about 45 degrees with upper and lower edges of the screen of the liquid crystal panel. With the TN type liquid crystal display having the polarizing plates whose axes of passage of light set as described above, the viewing direction in which the displayed images can be watched most clearly is slightly tilted from the direction normal to the screen toward the lower edge of the screen.
The liquid crystal displays are classified into a type in which images projected on a screen are observed from the projection side and a type in which a translucent back-projection type screen is used, and images projected on the screen from the back side thereof are observed from the front side thereof. In the back-projection type liquid crystal projector using the back-projection type screen, in order to widen a viewing angle for the images projected on the screen, a lenticular lens, in which many minute lenses are arranged in the form of stripes, is attached to a surface of the screen to diffuse light transmitted to the surface of the screen.
The lenticular lens is adapted to widen the viewing angle for projected images by the use of light diffusion function of each stripe lens in the direction of width thereof. Hence, the projected image seen on the screen on which the lenticular lens is attached is slightly extended in the direction in which the viewing angle is widened. If stripe lenses of the lenticular lens are formed vertically or horizontally, then the projected images will be observed without being distorted unnaturally. In general, the stripe lenses are formed vertically to widen the viewing angle horizontally. In the case of the horizontal stripe lenses, the viewing angle will be widened vertically.
As described above, the conventional back-projection type liquid crystal projector uses a direct-vision type liquid crystal display in which the light-falling polarizing plate and the image-forming polarizing plate on the light-outgoing plane have their axes of passage of light arranged to make an angle of about 45 degrees with upper and lower edges of the liquid crystal panel. Therefore, the image light projected on the back-projection type screen, on a surface of which the lenticular lens having vertical or horizontal stripe lenses is parallel formed, oscillates in the direction displaced by about 45 degrees from the direction of width of each stripe lens of the lenticular lens. This results in poor light transmittance of the screen and thus production of dark images on the viewing surface of the screen. This is due to the surface reflection of the lenticular lens.
The reflectance of incident light on the surface of each stripe lens of the lenticular lens is smallest when the incident light is so called P polarized light oscillating in the direction of width of the stripe lens. In the conventional back-projection type liquid crystal projector, the incident light on the back projection type screen strongly polarizes in the direction of about 45 degrees relative to the direction of width of each stripe lens of the lenticular lens. Hence, the surface reflectance of the lenticular lens becomes large and the light transmittance is thus reduced correspondingly. Consequently, the brightness of images seen on the screen reduces.
If each stripe lens of the lenticular lens were slanted by about 45 degrees with respect to the upper and lower edges of the screen, then the direction of width of the stripe lens could be suited to the oscillating direction of the incident light on the screen so as to increase the light transmittance. However, this would stretch the projected images on the slant and thus result in unnaturally distorted images.
It is accordingly an object of the present invention to provide a back-projection type display which, in spite of the use of a back-projection type screen on which a lenticular lens in which many vertical or horizontal stripe lenses are arranged in parallel is formed, makes it possible to see highly bright projected images on the viewing surface of the screen.